1. Field of the Invention
The invention is directed to the field of transient voltage surge suppression (TVSS) devices and more particularly to the protection of metal oxide varistors (MOV) used in such devices.
2. Description of the Prior Act
MOVs are commonly used as the transient suppression element of TVSS devices. Although they are capable of clamping large narrow voltage spikes by diverting transient currents, they generally are characterized by very low energy (watt-second) absorption capability. When their energy rating is exceeded they tend to explode in a violent fashion propelling metal fragments in all directions producing possible injury to persons and equipment nearby.
A conventional way of preventing this is to put a current rated fuse in series with the MOV. The fuse preferably is rated so that it will blow before the MOV energy rating is exceeded. Another approach utilizes a thermal fuse in direct thermal contact with a face of the MOV and is chosen such that it will be blow before the temperature rating of the MOV is exceeded.
In both of these cases the explosion of the MOV is prevented but the blowing of either type of fuse removes the transient suppression ability for the protected circuit because of the absence of the MOVs.